1. Field of the Invention
The invention relates to optical reading on a magnetic recording medium.
The following is the general principle of the magneto-optical effect (Kerr or Faraday effect): a thin layer of magnetic material, illuminated by a light beam, modifies the properties of this beam in a way that depends on the magnetic polarization of the layer.
Consequently, the writing of information elements on a magnetic tape is magnetic writing, but the reading is optical reading.
The nature of the modification of the light beam may be varied: it may entail the modification of the reflection coefficient, the introduction of a variable delay of optical phase or, again, a variation of the plane of polarization of a polarized light. It is the last-named phenomenon that is the easiest to use.
2. Description of the Prior Art
In practice, it is not possible to carry out a direct reading, by means of an optical beam, of the magnetic information elements recorded on the tape. A magneto-optical transducer, which constitutes the reading head of the system, is used. This transducer has a thin magnetic layer capable of giving rise to a magneto-optical effect. The tape, which bears magnetic information elements, moves past the head in its immediate vicinity and induces modifications of the magnetic flux in the thin layer of the head. It is towards this thin layer and not towards the band that a reading laser beam is directed.
A known magneto-optical reading system is built in the manner shown schematically in FIG. 1: a light source 10 (preferably a semiconductor laser diode) emits a beam 12 which is collimated by a collimator 14 and focused by a lens 16 on the reading head 18, and especially on the magneto-optical thin layer located in the vicinity of the magnetic tape 20 to be read. The beam is reflected by the thin layer, and the plane of polarization of the reflected light undergoes a rotation that depends on the magnetic polarization of the thin layer. This magnetic polarization itself depends on the direction of magnetic polarization of the portion of tape that is passing before the reading head. The reflected beam is focused by a lens 22 on a photosensitive head 24, in passing through a polarization analyzer 26. The electrical signal given by the photosensitive device depends on the rotation of polarization of the light, hence on the magnetic polarization of the tape at each instant. Naturally, a very high precision of relative positioning of these elements is necessary.